Currently, the requirement on performance of a panel display becomes higher due to development of multimedia technique and coming of information society. Organic electroluminescent display has a series of advantages such as autonomous luminescence, direct current driving at a low voltage, full solidification, wide view angle and being colorful. Its response speed is 1,000 times higher than LCD, while its manufacturing cost is lower than LCD having equal resolution. Thus, the organic electroluminescent display has broad prospects.
Research on organic electroluminescent display (also called organic light emitting diode, OLED) starts in 1960's. Electroluminescent phenomenon of single crystal of anthracene is firstly reported by Pope et al. (Pope M., Kallmann H. P. and Magnante R. J., Chem. Phys., 1963, 38, 2042), which preludes organic solid electroluminescence. In 1987, on the basis of summarizing previous researchers' work, the researcher C. W. Tang et al. (C. W. Tang, S. A. Vanslyke, Appl. Phys. Lett., 1987, 51, 913) in Kodak Company, U.S. proposes a design concept of structure of double layer and respectively selects triarylamine compounds and 8-hydroxyquinolinato aluminum complex (Alq3) having a good film-forming property for a hole transport layer and a luminescent layer (also acting as an electron transport layer), leading to an organic electroluminescent apparatus having high quantum efficiency (>1%), high efficiency (1.5 1 m/W), high luminance (>1000 cd/m2) and low driving voltage (<10V). In 1990, Burroughes et al. Nature, 1990, 347, 539-41) in Cavendish laboratory of University of Cambridge prepares a polymer electroluminescent apparatus by using polyphenylene vinylene (PPV) as a material for luminescent layer, and sets up another new field of light emitting apparatus, polymer thin film electroluminescent apparatus. These two significant progresses make the organic electroluminescent apparatus have potential to be a new generation of panel display.
The organic electroluminescent apparatus is composed of two opposite electrodes and an organic medium between the electrodes, the organic medium including a hole injection layer, a hole transport layer, a luminescent layer, an electron transport layer, a charge blocking layer and the like. It is generally believed that holes are often more than electrons in the OLED apparatus, causing imbalance of two carriers at interface of recombination and reducing luminance and efficiency of the apparatus. Meanwhile, redundant holes are easily transported into the electron transport layer and even a cathode, which accelerate aging of the apparatus and shorten life of OLED. Therefore, enhancing the injection and transport of electron become a subject being concerned and investigated extensively in the field. In addition to the cathode being highly efficient and stable, a hole blocking layer, the electron transport layer and the electron injection are disposed between the luminescent layer and the cathode, respectively performing functions of blocking holes to restrict excitons in the emissive region, transport electrons, and injecting electrons.
An electron transport material conventionally used in the organic electroluminescent apparatus is Alq3, but it has a lower electron mobility (at about 10−6 cm2/Vs). In order to enhance the electron transport property of the organic electroluminescent apparatus, much exploratory development has been made. Huang et al. employs nanometer cesium carbonate as an electron transport and injection material in the organic electroluminescent apparatus, thereby increasing luminous efficiency of the apparatus (Advanced Functional Materials, 2007, 17, 1966-1973). It is reported by LG Chemical Co. Ltd. that when benzimidazole, benzothiazole or benzoxazole compounds are used as an electron transport material in the organic electroluminescent apparatus, electron transport property of the apparatus is improved and turn-on voltage is reduced. (Chinese Patent Application No. 200680041587.4 with Publication Number CN 101305071A). When ammonium salt of derivative of terfluorene (abbreviated FFF-B1m4) synthesized by Cao Yong et al. (J. Am. Chem. Soc., 2008, 130(11), 3282-3283) is used a material for an electron injection layer, electron injection and transport of the apparatus are significantly improved, and electroluminescent efficiency is increased. Yang et al. also uses gold which is stable to air and various chemical corrosion as a cathode material of high efficient electron injection type, which improve electron injection ability of the organic electroluminescent apparatus (Organic Electronics, 2005, 6, 118-128). It is very important to develop stable and highly efficient electron transport material and/or electron injection material, thereby reducing turn-on voltage, improving efficiency of the apparatus, and increasing life of the apparatus.
A desirable electron transport material should have following characteristics: reversible electrochemical reduction reaction, suitable HOMO and LUMO levels, high electron mobility, good film-forming ability, high Tg, and preferably blocking holes. In terms of structure of compound, it is required that molecular configuration is close to plane, thereby enhancing π-π interaction among molecules when the molecules are stacked, while it is required that the structure of the molecule is not completely planar, thereby preventing the film-forming property from being affected by crystallization of the molecule; it is required that the molecule contains a structural unit lack of electron, thereby possessing good electron accepting ability; and the molecular weight is high enough to ensure high Tg and thus good thermal stability, while the molecular weigh can not be too high in order to facilitate film-forming by vacuum deposition.